This invention relates to non-aqueous electrolyte compositions for secondary (rechargeable) lithium battery cells and, more particularly, to electrolyte compositions that are capable of resisting decomposition normally resulting from oxidation which occurs in Li.sub.1+x Mn.sub.2 O.sub.4 /carbon cells during recharging under conditions of greater than about 4.5 V or 55.degree. C.
The advantages generally provided by rechargeable lithium batteries are often significantly overshadowed by dangers of the reactivity of lithium in cells which comprise lithium metal as the negative electrode. A more advanced and inherently safer approach to rechargeable lithium batteries is to replace lithium metal with a material capable of reversibly intercalating lithium ions, thereby providing the so-called "rocking-chair" battery in which lithium ions "rock" between the intercalation electrodes during the charging/recharging cycles. Such a Li metal-free "rocking-chair" battery may thus be viewed as comprising two lithium-ion-absorbing electrode "sponges" separated by a lithium-ion conducting electrolyte usually comprising a Li.sup.+ salt dissolved in a non-aqueous solvent or mixture of such solvents. Numerous such salts and solvents are known in the art, as evidenced in Canadian Patent Publication No. 2,022,191, dated Jan. 30, 1991.
The output voltage of a rechargeable lithium battery cell of this type is determined by the difference between the electrochemical potential of Li within the two intercalation electrodes of the cell. Therefore, in an effective cell the positive and negative electrode materials should be able to intercalate lithium at high and low voltages, respectively. Among the alternative materials that can effectively replace lithium metal as the negative electrode, carbon provides the best compromise between large specific capacity and good reversible cycling behavior. Such use of carbon, however, presents some detractions, such as loss of average output voltage and energy density, as compared to lithium metal, since the voltage of a Li.sub.x C.sub.6 negative electrode is always greater than that of a pure lithium negative electrode.
To compensate for the loss of voltage associated with the negative electrode, a strongly oxidizing intercalation material is preferably used as the positive electrode. Such an electrode material is the spinel phase Li.sub.1+x Mn.sub.2 O.sub.4, usually combined with a small amount of carbon black to improve electrical conductivity and provide the practical composite electrode, that can reversibly intercalate lithium at a voltage of 4.1 V vs. Li. Use of such a strongly oxidizing intercalation material as positive electrode, however, introduces a further concern, namely, the risk of electrolyte decomposition from oxidation at the higher operating voltages, i.e. greater than about 4 V. For instance, since the voltage of the Li.sub.1+x Mn.sub.2 O.sub.4 /Li couple is about 4.1 V, one should charge the cell up to a voltage of about 4.5 V in order to take full advantage of this redox system. As a result, the electrolyte in such a cell must be stable over a voltage window extending above 4.5 V to about 5.0 V. Also, when used in the noted "rocking chair" cells, the electrolyte compositions must be stable down to about 0 V with respect to a composite carbon negative electrode, e.g., petroleum coke combined with about 1-5% of each of carbon black (Super-S) and an inert binder.
Presently-used intercalation electrolytes, e.g., a 1M solution of LiClO.sub.4 in a 50:50 mixture of ethylene carbonate (EC) and diethoxyethane (DEE) such as described in U.S. Pat. No. 5,110,696, when employed in a Li.sub.1+x Mn.sub.2 O.sub.4 /C cell, will begin to oxidize at about 4.5 V at room temperature and as low as about 4.3 V at temperatures in the range of 55.degree. C. Thus, to operate such a cell in the higher temperature ambient, one must reduce the charging cut-off voltage to a level below about 4.3 V in order to avoid electrolyte oxidation. Because of this lower cut-off voltage, the available capacity of the cell at about 55.degree. C. is only 75% of that at room temperature.
When cells comprising these previously-available electrolytes are cycled to a voltage even slightly greater than 4.3 V, electrolyte oxidation occurs. Although small, this oxidation can jeopardize the capacity, cycle life, and safety of the battery cell. For example, the electrode oxidation reaction consumes part of the charging current which is then not recovered when discharging the cell, resulting in a continuous loss in the cell capacity over subsequent cycles. Further, if during each charge a small part of the electrolyte is consumed, excess electrolyte must be included when the cell is assembled. This in turn results in less active material for a constant volume battery body and consequently less initial capacity. In addition, the oxidation of the electrolyte often generates solid and gaseous byproducts, the solid of which build up a passivating layer on the particles of the active material, increasing the polarization of the cell and lowering the output voltage. Simultaneously, and more importantly, the gaseous byproducts increase the internal pressure of the cell, thereby increasing the risk of explosion and leading to unsafe and unacceptable operating conditions.